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Limits on Fluorescence Detected Circular Dichroism of Single Helicene Molecules

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Limits on Fluorescence Detected Circular Dichroism of Single Helicene Molecules 6213 2009, 113, 6213–6216 Published on Web 05/13/2009 Limits on Fluorescence Detected Circular Dichroism of Single Helicene Molecules Yiqiao Tang,† Timothy A. Cook,‡,§ and Adam E. Cohen*,†,‡ Department of Physics, HarVard UniVersity, Cambridge, Massachusetts 02138, and Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138 ReceiVed: April 19, 2009; ReVised Manuscript ReceiVed: May 6, 2009 Fluorescent imaging of single helicene molecules is applied to study the optical activity of chiral fluorophores. In contrast to the previous report by Hassey et al. (Science 2006, 314, 1437), the dissymmetry factors of single chiral fluorophores are found not to differ significantly from the bulk value of |g| < 10-4 at 457 nm. Linear dichroism and birefringence of the dichroic mirror inside the fluorescence microscope change the polarization state of the incoming laser beam significantly; i.e., circular polarized light sent into the microscope becomes highly elliptically polarized after reflection from the dichroic mirror. Compensation for this effect should be made to avoid artifacts brought by linear dichroism in single immobilized molecules. Single-molecule measurements have been used to probe the H )-µ · E - θ:∇E - m · B states and dynamics of a huge variety of biophysical and condensed matter systems,2-4 and often yield information that where µ is the electric dipole operator, θ is the electric is missed by bulk, ensemble-averaged techniques. In particular, quadrupole operator, and m is the magnetic dipole operator; E measurements at ambient and low temperatures provide detailed is the electric field, and B is the magnetic field. The rate of information about the interactions between single fluorescent excitation from an initial state i to a final state f involves the 2 molecules and their local environment. The heterogeneously expression |〈f|H|i〉| , as specified by Fermi’s golden rule. This 2 broadened lines visible in bulk often resolve into much narrower expression contains a term proportional to |µif| , as well as cross- single-molecule lines, which show spectral diffusion,5,6 blinking,7 terms containing µ and θ or µ and m (the remaining terms are 2 and polarization fluctuations8,9 that indicate complex interactions small enough to be neglected). The term containing |µif| with the host. represents electric dipole absorption, and is not sensitive to molecular chirality. The signs of the two cross-terms, however, Chiral molecules of a single enantiomer show differential depend on molecular chirality, so these terms are responsible absorption of left and right circularly polarized light (CPL). The for circular dichroism. Both cross-terms depend on the orienta- degree of circular dichroism (CD) at a particular frequency is tion of the molecule relative to the incident field. Upon averaging measured by the g-value: over all molecular orientations, the electric quadrupole contribu- tion to the differential absorption averages to zero while the Publication Date (Web): May 13, 2009 | doi: 10.1021/jp903598t 13 + - - magnetic dipole term remains. Thus, electric quadrupole ≡ 2(Γ Γ ) transitions do not contribute to bulk CD, while magnetic dipole Downloaded by HARVARD UNIV on September 3, 2009 | http://pubs.acs.org g + - (Γ + Γ ) transitions do contribute. In light of the increased information available from single- molecule measurements, it is interesting to compare the chi- ( where Γ is the rate of excitation in right (+) or left (-) CPL. roptical response of a single chiral molecule to the ensemble- The g-value varies between -2 e g e 2. For most small organic averaged response. One can measure chiroptical effects in small - molecules, peptides, nucleic acids, and sugars, |g| < 10 3 in the samples by using fluorescent chiral molecules.15,16 The rate of visible.10 The smallness of g arises from the small size of fluorescence emission is proportional to the rate of excitation, molecules relative to the helical pitch of CPL: the electromag- and thus, fluorescence detected circular dichroism (FDCD) netic field undergoes a nearly imperceptible twist over a distance provides a possible probe of chiroptical effects at the single- of molecular dimensions. molecule level. Single-molecule FDCD might differ from bulk At the quantum mechanical level, CD arises through an FDCD for several reasons: interference between electric dipole and either electric quadru- (1) Electric quadrupole and magnetic dipole matrix elements pole or magnetic dipole transitions.11 The Hamiltonian relevant of orientationally fixed single molecules may differ from their 13,17 to CD for a chiral molecule in an electromagnetic field is:12-14 rotationally averaged values in bulk systems. (2) Local interactions with the host could modify the oscillator * To whom correspondence should be addressed. E-mail: cohen@ strengths or frequencies of the electric quadrupole or magnetic chemistry.harvard.edu. dipole transitions, to induce molecule-to-molecule variations in † Department of Physics. CD, i.e., heterogeneous broadening of CD lines. ‡ Department of Chemistry and Chemical Biology. § Current address: School of Law, University of Virginia, Charlottesville, The smallness of the electric quadrupole and magnetic dipole VA 22903. contributions to the Hamiltonian is purely geometrical in origin, 10.1021/jp903598t CCC: $40.75 2009 American Chemical Society 6214 J. Phys. Chem. A, Vol. 113, No. 22, 2009 Letters a consequence of the small size of a molecule relative to the wavelength of light. Orientational averaging reduces the strength of the electric dipole/magnetic dipole interference by a factor of 3 relative to its peak value.13 Electric quadrupole transitions average to zero in bulk, so bulk measurements provide no guidance on their magnitude. Geometrical considerations, however, indicate that the electric quadrupole term should be of the same order of magnitude as the magnetic dipole term. Density functional simulations of single helicene molecules support the prediction that single-molecule g-values are of the order of 10-3, of the same order as in bulk at the same excitation wavelength.18 Nonetheless, theoretical models may be wrong, so there was considerable interest in the report from Hassey et al.1 that single molecules of bridged triarylamine helicenes show g-values ranging from +2to- 2, while in bulk the two enantiomers have |g| < 10-4 at a probe wavelength of 457 nm. Furthermore, they reported that both enantiomers of this compound show very similar broad distributions of single-molecule g-values, and that the wavelength dependence of the single-molecule g-values does not correspond to the bulk, even when averaged over many single molecules.18 We attempted to replicate the experiments of Hassey et al. and initially saw similar broad distributions of g-values. However, after correcting for the linear birefringence and linear dichroism inherent to the dichroic mirrors used in single- molecule FDCD experiments, the distribution of g-values collapses to a sharp distribution around g ) 0. The uncertainty in the measurements does not allow us to distinguish the g-values of opposite enantiomers. Figure 1. Bridged triarylamine helicenes. (a) Structure of the P-2 enantiomer. The camphanate group is used as a chiral resolving agent Methods and renders the M-2 and P-2 species technically diastereomers. However, the camphanate group has no detectable effect on the optical Triarylamine helicenes M-2 and P-2 (Figure 1a) were properties of either species. (b) Circular dichroism spectrum of M-2 synthesized following literature procedures19,20 with modifica- and P-2, measured with a JASCO CD spectrometer. The spectrum of tions. (2-Methoxyphenyl)-naphthalenylamine was synthesized the P-2 was scaled by a constant factor to correct for a slight difference in concentration between the two samples. using (1,1′-bis(diphenylphosphino)ferrocene)PdCl2 · CH2Cl2 as the amination catalyst,21 and deprotection of the helicene methyl 22 ether was accomplished using BBr3. The molecules we (FM600QM Thorlabs). The LCVR is driven with a 2 kHz square prepared are identical to the ones Hassey et al. used in ref 1. wave. An arbitrary state of ellipticity is generated by adjusting The products were characterized by 1H NMR, 13C NMR, and the LCVR drive voltage. We have mirrors between the polarizer Publication Date (Web): May 13, 2009 | doi: 10.1021/jp903598t mass spectrometry; all data match the literature spectra. The and the microscope, to avoid introducing spurious phase shifts circular dichroism (Figure 1b) and fluorescence (not shown) onto the beam. The only reflection after the initial polarizer is Downloaded by HARVARD UNIV on September 3, 2009 | http://pubs.acs.org spectra are identical to earlier reports.19 Both enantiomers show off the dichroic mirror. strong FDCD in bulk when excited with light at 355 nm, with A lens with a focal length of 15 cm (LA1433-A Thorlabs) g-values of +8.0 × 10-3 for M-2 and -8.1 × 10-3 for P-2. At brings the light to a focus at the back focal plane of the objective. 457 nm, the wavelength used in the experiments of Hassey et After passing through the lens, the light is deflected upward by al., the g-values are |g| < 10-4. a dichroic mirror. We performed experiments using either of Following the procedure of Hassey et al., we made solutions two dichroics: DC1, 460 nm long pass; DC2 (Olympus DM500), of the helicenes at 10-8 M in methanol and drop-cast these 500 nm long pass. Our objective lens is a 60×, N.A. 1.45, oil solutions onto a Zeonor film (ZF-14 ZEONEX/ZEONOR). A immersion, plan apochromat (1-U2B616 Olympus). Fluores- fused silica coverslip was cleaned in Piranha solution (3:1 cence from the sample is collected by the same objective, passed H2SO4:H2O2, Caution: highly corrosive). The Zeonor film was through the dichroic mirror, and separated from the excitation placed, helicene side down, onto the coverslip, and the samples light by a 470 nm long-pass emission filter (HQ470LP Chroma).
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